In line with rapid increase in use of fossil fuels, demand for alternative energy or clean energy is increasing. Thus, the field of power generation and electrochemical electricity storage is most actively studied.
As a representative example of electrochemical devices using electrochemical energy, secondary batteries are currently used and use thereof is gradually expanding.
Recently, as technology for portable devices, such as portable computers, portable phones, cameras, and the like, continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, research on lithium secondary batteries having high energy density, high operating potential, long cycle lifespan and low self-discharge rate has been underway and such lithium secondary batteries are commercially available and widely used.
In addition, as interest in environmental problems is increasing, research into electric vehicles, hybrid electric vehicles, and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is underway. As a power source of electric vehicles, hybrid electric vehicles, and the like, nickel-metal hydride secondary batteries are mainly used. However, research into lithium secondary batteries having high energy density and high discharge voltage is actively carried out and some of the lithium secondary batteries are commercially available.
Conventional typical lithium secondary batteries use graphite as a negative electrode active material and charge and discharge thereof are performed by repeating a process that lithium ions of a positive electrode are inserted into and eliminated from a negative electrode. Even though there is a difference in the theoretical capacity of the battery depending upon kinds of electrode active materials, the charge/discharge capacity of the battery usually decreases as the number of charge/discharge cycle increases.
The primary cause of such a phenomenon is a failure to sufficiently fulfill functions of the electrode active material due to separation between the electrode active materials and/or between the electrode active material and current collector, resulting from volume changes of electrodes occurring during repeated charge/discharge cycles of the battery. Further, since the lithium ions inserted into the negative electrode are not normally released from the negative electrode during the insertion and elimination process, the active points of the negative electrode are decreased as the number of charge/discharge cycle increases. Consequently, further increase in the number of charge/discharge cycles also leads to decrease of the charge/discharge capacity and deterioration of life characteristics in the battery.
Thus, there is an urgent need in the art to study a binder and an electrode material that may have strong adhesive strength so as to prevent separation between electrode active material components or separation between an electrode active material and a current collector when manufacturing an electrode and may have strong physical properties so as to achieve structural stability of an electrode by controlling volume expansion of an electrode active material due to repeated charge/discharge and, accordingly, enhance battery performance.
A conventional solvent-based binder, i.e., polyvinylidene fluoride (PVdF), does not meet such requirement and thus, recently, a method of using binders prepared by preparing emulsion particles by aqueous polymerization of styrene-butadiene rubber (SBR) and mixing the emulsion particles with a neutralizing agent and the like has been proposed and is currently commercially available. These binders are eco-friendly and used in a small amount and thus may increase battery capacity. However, such a case also exhibits improved adhesive durability due to rubber elasticity but does not exhibit dramatically improved adhesive strength.
Therefore, there is an urgent need to develop a binder that enhances cycle characteristics of a battery, imparts structural stability to an electrode, and has high adhesive strength.